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        <span itemprop="articleBody"><p>通常Android应用程序要与远程服务器进行信息交换．最简单的方式是以HTTP协议作为基础来传输信息．在某些环境下使用HTTP协议是非常有用的，比如从远程服务器下载图片或者上传二进制数据到服务器．Android应用程序通过执行GET和POST请求(request)来发送数据．在这篇帖子，我想要分析分析一下如何使用<code>HttpURLConnection</code>与远程服务器进行通信．</p>
<h3 id="GET和POST请求">GET和POST请求</h3><p>GET和POST请求在HTTP协议中是基块(the base blocks)。为了生成这类请求我们首先需要打开一个指向远程服务器的链接:<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">HttpURLConnection con = (HttpURLConnection) (<span class="keyword">new</span> URL(url)).openConnection();</span><br><span class="line">con.setRequestMethod(<span class="string">"POST"</span>);</span><br><span class="line">con.setDoInput(<span class="keyword">true</span>);</span><br><span class="line">con.setDoOutput(<span class="keyword">true</span>);</span><br><span class="line">con.connect();</span><br></pre></td></tr></table></figure></p>
<p>首先，我门通过<code>url</code>参数构建一个<code>Url</code>对象，通过这个对象的<code>openConnection</code>方法得到<code>HttpURLConnection</code>,代码第二行我们设置请求的方式为Post，最后connect到服务器。</p>
<p>一旦我们持有一个打开了的链接我们就可以使用<code>OutputStream</code>进行写入我们的数据了。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">con.getOutputStream().wirte((<span class="string">"name="</span>+name).getBytes());</span><br></pre></td></tr></table></figure></p>
<p>对于我们已经知道的参数我们可以使用键值对来写入。</p>
<p>最后一步是使用<code>InputStream</code>读取响应数据。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">InputStream ins = con.getInputStream();</span><br><span class="line"><span class="keyword">byte</span>[] b = <span class="keyword">new</span> <span class="keyword">byte</span>[<span class="number">1024</span>];</span><br><span class="line"><span class="keyword">while</span>(ins.read(b) != -<span class="number">1</span>)</span><br><span class="line">    buffer.append(<span class="keyword">new</span> String(b));</span><br><span class="line">con.disconnect();</span><br></pre></td></tr></table></figure></p>
<p>到目前为止所有的工作都非常简单，但是需要记住的一点是：使用<code>HTTP</code>链接是一个耗时的操作，有时它可能需要很长的时间，因此我们不能把它放在UI线程里运行，否则我们会得到一个ANR(Application Not Responding)对话框。</p>
<p>解决这个问题的方法是我们可以使用<code>AsyncTask</code><br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="class"><span class="keyword">class</span> <span class="title">SendHttpRequestTask</span> <span class="keyword">extends</span> <span class="title">AsyncTask</span>&lt;<span class="title">String</span>, <span class="title">Void</span>, <span class="title">String</span>&gt;</span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="annotation">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">protected</span> String <span class="title">doInBackground</span><span class="params">(String... params)</span></span>&#123;</span><br><span class="line">	String url = params[<span class="number">0</span>];</span><br><span class="line">	String name = params[<span class="number">1</span>];</span><br><span class="line">	String data = sendHttpRequest(url, name);</span><br><span class="line">	<span class="keyword">return</span> data;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="annotation">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">onPostExecute</span><span class="params">(String result)</span></span>&#123;</span><br><span class="line">	edtResp.setText(result);</span><br><span class="line">	item.setActionView(<span class="keyword">null</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<h3 id="从服务器下载数据">从服务器下载数据</h3><p>一个最普遍的情景是应用程序从远程服务器下载一些数据。我们假设要从服务器下载一张图片，在这例子里我们依旧使用<code>AsyncTask</code>来完成我们的操作，代码如下：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">byte</span>[] downloadImage(String imgName)&#123;</span><br><span class="line">    ByteArrayOutputStream baos = <span class="keyword">new</span> ByteArrayOutputStream();</span><br><span class="line">    <span class="keyword">try</span>&#123;</span><br><span class="line">	System.out.println(<span class="string">"URL ["</span>+url+<span class="string">"] - Name ["</span>+imgName+<span class="string">"]"</span>);</span><br><span class="line"></span><br><span class="line">	HttpURLConnection con = (HttpURLConnection) (<span class="keyword">new</span> URL(url)).openConnection();</span><br><span class="line">	con.setRequestMethod(<span class="string">"POST"</span>);</span><br><span class="line">	con.setDoInput(<span class="keyword">true</span>);</span><br><span class="line">	con.setDoOutput(<span class="keyword">true</span>);</span><br><span class="line">	con.connect();</span><br><span class="line">	con.getOutputStream.write((<span class="string">"name="</span> + imgName).getBytes());</span><br><span class="line"></span><br><span class="line">	InputStream is = con.getInputStream();</span><br><span class="line">	<span class="keyword">byte</span>[] b = <span class="keyword">new</span> <span class="keyword">byte</span>[<span class="number">1024</span>];</span><br><span class="line"></span><br><span class="line">	<span class="keyword">while</span>( is.read(b) != -<span class="number">1</span>)</span><br><span class="line">	    baos.write(b);</span><br><span class="line"></span><br><span class="line">	con.disconnect();</span><br><span class="line">    &#125;<span class="keyword">catch</span>(Throwable t)&#123;</span><br><span class="line">	t.printStackTrace();</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> baos.toByteArray();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>这个方法以下面这样的方式调用：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="class"><span class="keyword">class</span> <span class="title">SendHttpRequestTask</span> <span class="keyword">extends</span> <span class="title">AsyncTask</span>&lt;<span class="title">String</span>, <span class="title">Void</span>, <span class="title">String</span>&gt;</span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="annotation">@Override</span></span><br><span class="line">    <span class="keyword">protected</span> <span class="keyword">byte</span>[] doInBackground(String...params)&#123;</span><br><span class="line">	String url = params[<span class="number">0</span>];</span><br><span class="line">	String name = params[<span class="number">1</span>];</span><br><span class="line"></span><br><span class="line">	HttpClient client = <span class="keyword">new</span> HttpClient(url);</span><br><span class="line">	<span class="keyword">byte</span>[] data = client.downloadImage(name);</span><br><span class="line"></span><br><span class="line">	<span class="keyword">return</span> data;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="annotation">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">onPostExecute</span><span class="params">(<span class="keyword">byte</span>[] result)</span></span>&#123;</span><br><span class="line">	Bitmap img = BitmapFactory.decodeByteArray(result, <span class="number">0</span>, result.length);</span><br><span class="line">	imgView.setImageBitmap(img);</span><br><span class="line">	item.setActionView(<span class="keyword">null</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<h3 id="使用MultipartRequest上传数据到服务器">使用MultipartRequest上传数据到服务器</h3><p>这是在处理<code>http</code>链接里最复杂的部分，本地的<code>HttpUrlConnection</code>不处理这类请求,一个Android应用程序上传一些二进制数据到服务器时它会发生。<code>MultipartRequest</code>类主要对文件上传进行操作，在上传时，编码格式为<code>enctype=&quot;multipart/form-data&quot;格式，以二进制方式提交，提交方式为post方式。以上传一张图片为例子，在种情况下请求会有点复杂，因为一个“一般”的请求满足不了我门的需求，因此我们创建一个</code>MultipartRequest`.</p>
<p><code>MultipartRequest</code> 是一个通过像参数和二进制数据这样不同的部分构造的请求。</p>
<p>首先第一步是打开一个链接，通知(informing)服务器我们要发一些二进制数据。<br><figure class="highlight"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">public void connectForMultipart() throws Exception&#123;</span><br><span class="line">    con = (HttpURLConnection) (new URL(url)).openConnection();</span><br><span class="line">    con.setRequestMethod("POST");</span><br><span class="line">    con.setDoInput(true);</span><br><span class="line">    con.setDoOutput(true);</span><br><span class="line">    con.setRequestProperty("Connection", "Keep-Alive");</span><br><span class="line">    con.setRequestProperty("Content-Type", "multipart/form-data"; boundary=" + boundary);</span><br><span class="line">    con.connect();</span><br><span class="line">    os = con.getOutputStream();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>在第７行我们指定了请求的<code>content-type</code>和另一个字段<em>boundary</em>,　这个字段是一个字符序列用于分隔不同的部分。</p>
<p>对于每个部分，如果它是一个文本像提交的参数或者是一个文件（二进制数据）就需要指定想要添加的部分。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">addFormPart</span><span class="params">(String paramName, String value)</span><span class="keyword">throws</span> Exception</span>&#123;</span><br><span class="line">    writeParamData(paramName, value);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">writeParamData</span><span class="params">(String paramName, String value)</span> <span class="keyword">throws</span> Exception</span>&#123;</span><br><span class="line">    os.wirte( (delimiter + boundary + <span class="string">"\r\n"</span>).getBytes());</span><br><span class="line">    os.write( <span class="string">"Content-Type: text/plain\r\n"</span>.getBytes());</span><br><span class="line">    os.wirte( (<span class="string">"Content-Disposition: form-data; name=\""</span> +paramName + <span class="string">"\"\r\n"</span>).getBytes());</span><br><span class="line">    os.write( (<span class="string">"\r\n"</span> + value + <span class="string">"\r\n"</span>).getBytes());</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>这里：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> String delimiter = <span class="string">"--"</span>;</span><br><span class="line"><span class="keyword">private</span> String boundary = <span class="string">"SwA"</span>+ Long.toString(System.currentTimeMillis())+<span class="string">"SwA"</span>;</span><br></pre></td></tr></table></figure></p>
<p>使用下面的方式添加文件部分：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">addFilePart</span><span class="params">(String paramName, String fileName, <span class="keyword">byte</span>[] data)</span> <span class="keyword">throws</span> Exception</span>&#123;</span><br><span class="line">    os.write( (delimiter + boundary + <span class="string">"\r\n"</span>).getBytes());</span><br><span class="line">    os.write( (<span class="string">"Content-Disposition: form-data; name=\""</span>+ paramName + <span class="string">"\"; filename=\""</span> + fileName + <span class="string">"\"\r\n"</span>).getBytes());</span><br><span class="line">    os.write( (<span class="string">"Content-Type: application/octet-stream\r\n"</span>).getBytes());</span><br><span class="line">    os.write( (<span class="string">"Content-Transfer-Encoding: binary\r\n"</span>).getBytes());</span><br><span class="line">    os.write(<span class="string">"\r\n"</span>.getBytes());</span><br><span class="line"></span><br><span class="line">    os.write(data);</span><br><span class="line">    os.write(<span class="string">"\r\n"</span>.getBytes());</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>因此，使用在我们的例子里：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="class"><span class="keyword">class</span> <span class="title">SendHttpRequestTask</span> <span class="keyword">extends</span> <span class="title">AsyncTask</span>&lt;<span class="title">String</span>, <span class="title">Void</span>, <span class="title">String</span>&gt;</span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="annotation">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">protected</span> String <span class="title">doInBackground</span><span class="params">(String... params)</span></span>&#123;</span><br><span class="line">	String url = params[<span class="number">0</span>];</span><br><span class="line">	String param1 = params[<span class="number">1</span>];</span><br><span class="line">	String param2 = params[<span class="number">2</span>];</span><br><span class="line">	Bitmap b = BitmapFactory.decodeResource(UploadActivity.<span class="keyword">this</span>.getResources(), R.drawable.logo);</span><br><span class="line"></span><br><span class="line">	ByteArrayOutputStream baos = <span class="keyword">new</span> ByteArrayOutputStream();</span><br><span class="line">	b.compress(CompressFormat.PNG, <span class="number">0</span>, baos);</span><br><span class="line"></span><br><span class="line">	<span class="keyword">try</span>&#123;</span><br><span class="line">	    HttpClient client = <span class="keyword">new</span> HttpClient(url);</span><br><span class="line">	    client.connectForMulitipart();</span><br><span class="line">	    client.addFormPart(<span class="string">"param1"</span>, param1);</span><br><span class="line">	    client.addFormPart(<span class="string">"param2"</span>, param2);</span><br><span class="line">	    client.addFilePart(<span class="string">"file"</span>, <span class="string">"logo.png"</span>, baos.toByteArray());</span><br><span class="line">	    client.finishMultipart();</span><br><span class="line">	    String data = client.getResponse();</span><br><span class="line">	&#125;<span class="keyword">catch</span>(Throwable t)&#123;</span><br><span class="line">	    t.printStackTrace();</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="annotation">@Override</span> </span><br><span class="line">	<span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">onPostExecute</span><span class="params">(String data)</span></span>&#123;</span><br><span class="line">	    item.setActionView(<span class="keyword">null</span>);</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
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